I'm five minutes into my first Parkour session, and already I'm wondering what I've let myself in for. It never usually takes long for me to start perspiring at the best of times – walking down the street, waiting in queues, moving from the sitting to standing position – but with a full two hours of exercise still to go it can't be a good thing.

Fortunately the warm-up proves the toughest work of the night. By the time we reach the vigorous cool-down an hour-and-a-half later my muscles have given up the ghost, and no matter how many times the instructor tells me to crunch it isn't happening. I slouch there like a moist pink jelly, eyes slightly glazed and a faraway smile on my face. And that's the surprising thing – I've thoroughly enjoyed myself.

You might know Parkour as freerunning, brought to national attention by those BBC idents in 2002. It is, according to a press release, "a way of training one's body and mind in order to be as completely functional, effective and liberated as possible in the physical realm, and a way of thinking based on rigorous self-discipline, autonomous action and self-will". All of which sounds rather airy-fairy to me. What I learned was that, unlike so many gym classes that manage one but not the other, Parkour is exhausting and pretty good fun.

That warm-up excepted, of course. Not having done a great deal of physical exercise for the best part of six months (a laziness- rather than injury-related lay-off) it was a real challenge. Press-ups interspersed with sprints and vertical leaps mixed with reverse monkey-walking (a backwards crawl that makes the shoulders sing under the strain) got the pulse raised and the muscles warm. Twenty minutes in and my T-shirt is already sweat-spattered.

Then out comes the apparatus. This, you see, is a Parkour Academy class, an indoor session for men and women that is aimed as an introduction to the discipline as well as a conditioning class for those looking for the extra few muscles needed to take on the bigger jumps. Horses, vaults, benches and mats are arranged around the sports hall and the group is split into four, ranging from complete novices (me) to the seasoned veterans (several of whom are young enough to be my children, or at least a niece or nephew).

The novices begin with what appears to be an assault course of squidgy horses, which are well over waist height on my near-6ft frame. It's an intimidating prospect, but Chris Keighley, the class instructor and a man with a back more V-shaped than a Red Arrows formation, explains the principles of body shape and the ideal way to hop over the obstacles. He emphasises that there's no set rule. The flow of movement is the thing, regardless, at this stage at least, of how aesthetically pleasing it is.

This is just as well, as my first attempt is a messy stop-start effort that resembles a sputtering tap more than a flowing stream. The second, though, is much better, as is the third and it's from this moment that the pleasure of movement starts to hit home. It's not until it's time to switch to a new section that I realise my legs have grown weary and my feet are clipping the tops of the hurdles.

But it's not too much of a problem, because each section targets different muscle groups. There's a leap onto a bar, a set of static exercises to improve the muscles needed for the fun stuff, and what counts for a novice as a death-defying leap of faith from a low table to the top of a tall pommel horse. It's not quite up there with the building-hopping stuff, but pretty exhilarating nevertheless.

By this stage, I was beginning to feel rather pleased with myself. Then I glanced at the seasoned veterans/youths group. Their smoothness as they combat the obstacles is remarkable, bringing to mind a river running over smooth rocks. They make Parkour look like a cross between ballet and break-dancing. My pride gets pegged back a couple of notches. (After the class has finished, I hobble over to the tube station to find some of my class-mates trying out a few moves on the platform chairs. It highlights what I'd noticed as we sweated together earlier on – the class attracts a wide range of people, from hoodie-kids who are tweaking their technique, to middle-class gym-members looking for a new workout.) But it is, after all, all relative and it's surprising just what a buzz you can get from landing on a balance beam without wobbling off. Really.

Like all the best forms of exercise, it comes as a disappointment when it's time to stop. The cool down focused on the stomach muscles and, for someone like me whose abs only live in the imagination, it was a step too far. Others just about managed to squeeze a few more sit-ups into their routine. It'll be a while before I'm trying this sort of stuff out (but, let's face it, he's clearly a lunatic) or joining these chaps, but for a fitness class with a bit of a difference, I'd recommend it to anyone.